Vehicles and the like including a diesel engine that emits nitrogen oxides (hereinafter referred to as NOx) employ an exhaust gas purification system (hereinafter referred to as SCR system) using a selective catalytic reduction (SCR) catalyst to remove NOx. The SCR system includes a spray nozzle and the SCR catalyst. The spray nozzle sprays aqueous urea into an exhaust pipe, in which an exhaust gas from the engine flows. The SCR catalyst is disposed downstream of the spray nozzle. In the SCR system, the aqueous urea is sprayed to a high-temperature exhaust gas flowing through the exhaust pipe upon the discharge of exhaust gas from the engine to the exhaust pipe. The heat of the exhaust gas results in hydrolysis of urea to ammonia. The resulting ammonia flows downstream through the exhaust pipe together with the exhaust gas and then reaches the SCR catalyst. The SCR catalyst promotes the chemical reaction between ammonia and NOx in the exhaust gas, and NOx is converted into water and nitrogen.
In the SCR system, the spray nozzle is connected to an aqueous urea tank through a pipe, and the aqueous urea in the tank is supplied to the spray nozzle through the pipe as appropriate. In the SCR system, however, since the freezing point of the aqueous urea is approximately −11° C., the aqueous urea in the pipe is frozen and cannot be supplied to the spray nozzle in cold climate areas. To solve this problem, a technique that prevents freezing of the aqueous urea in the pipe has been developed. For example, Patent Literature 1 discloses a liquid transporting pipe in which a connecting portion between a pipe joint and a pipe and a pipe adjacent to the connecting portion are covered by a dissipation preventing cover, and a cord heater is inserted in the pipe.